Quantitative spectroscopic analysis and apparatus therefor



June 2, 1936. E J Tm I 2,043,053

QUANT ITAT IVE SI ECTROSCOP I C ANALYS I S AND APPARATUS THEREFOR FiledAug. 29, 1932 s Shets-Sheet 1 J1me 19361- E. J. MARTIN I 2,043,053

QUANTITATIVE SPECTROSCOPIC ANALYSIS AND APPARATUS THEREFOR Filed Aug;29, 1932 s Sheets-Sheet 2 61am! mafia June 2, 1936. E. J. MARTHN 3 9QUANTITATIVE SPECTROSCOPIC ANALYSIS AND APPARATUS THEREFOR Filed Aug.29, 193 5 Sheets-Sheet 3 (U AMP Patented June 2, 1936 PATENT orricr.

QUANTITATIVE SPECTROSCOPTG ANALYSIS .AND APPARATUS THEREFOR Edward J.Martin, Ferndale', Mich, assignor, by

mesne assignments, to General Motors Corporation, Detroit, Mich., acorporation of Delaware Application August 29, 1932, Serial No. 630,891

3 Claims.

This application for patent presents the results of a long period ofresearch having as its object a simple, practical method of quantitativespectroscopic analysis. The demand for the method came frommetallurgists who encountered difiiculty in controlling composition ofmelts owing to the fact that the methods of chemical analysis were soslow that by the, time the analysis was completed the composition of themelt had so changed that the proper time for pouring had passed. Themetallurgists desired a speedy accurate method that would overcome thisdifliculty.

The method described in this application has proven to be rapid,accurate and practical, and fills the need which led to theinvestigation. Like prior methods of analyzing materials by study of thespectrum, it is of almost universal application, and aiiords a degree ofaccuracy in some cases beyond that obtainable by chemical methods. Italso permits measurements of quantities of materials diflicult orimpossible to separate from other ingredients by chemical methods.

One of the chief advances of my method con- I sists in the incorporationof photographic plate calibration markers in the spectrogram itself.Such calibration marking is essential where photographic methods ofmeasurement are employed, due to the fact that these factors materiallyalter the opacity of the images of the lines of the spectrum and preventthe opacity from being a true measure of the intensity of thecorrespond- I ing line in the spectrum. Since the opacity of ofgraduated opacity has been obtained by projecting light of uniformbrightness through slots of graduated area, and focusing the bands oflight on equal areas of the plate. This method requires an additionaloperation in placing the pattern on the photographic plate. According tomy method the exposure of different portions of each of the spectrallines is varied in known ratio by employing suitable means such as amovable shutter having cut-off portions similarly graduated. By thismeans I attain both simplicity and greater accuracy.

I have likewise substantially improved the method of measuring theopacity of the lines on the spectrogram. This has been accomplished byinserting the spectrogram in a projecting lantern that casts an enlargedimage of the spec-- trogram on a screen. The screen is provided with anaperture which receives light coming through the lines whose opacitiesare to be measured. A suitable light measuring device is arranged toreceive the light coming through the aperture. By this means the opacityof each of the segments of a selected line may be measured and plottedagainst the exposures which, in turn, are proportional to the quantitiesor intensities of light in the corresponding lines of the spectrum. Bynow measuring the opacity of one portion of another spectral line andcomparing the exposure of said portion with the exposure of thecorresponding part of the first line a ratio of exposures or lightquantities is obtained which is peculiar to the particular concentrationof the second element in the material. By compiling the results ofsimilar tests of elements of known composition, it is possible toevaluate any subsequent light quantity ratio in terms of percentage ofthe element, thereby performing the analysis.

The complete process requires only the photographing of a portion of thespectrum of the material to be analyzed, projecting the image of thephotograph on a screen, making a few opacity measurements,not to exceedsix. according to my preferred method,-to obtain the desired lightquantity ratio .which may then be evaluated from data previouslycompiled. The apparatus may be so designed that an unskilled person mayperform the operations. The expense of the analysis is confined to thepreparation of the arc source if that method of excitation be employed,the cost of the small portion of sensitized material used to receive thespectrogram, anda few minutes of the time of an unskilled operator.

In developing this method, improvements have been made in the apparatusas well. My invention includes improvements in the spectrograph toincorporate the calibration marker in the spectogram; improvements indensitometers for measuring the opacities of the selected lines;improvements in spectrograms by incorporating the opacity calibrationmarker therein; improve? I shutter which is an important feature of myinvention.

Figure 5 is a view, partly is section, showing the preferred form ofcathode used in producing the arc.

Figure 6 is a perspective view of the projection type densitorneter asset up in the laboratory for use in accordance with my invention.

Figure '7 is a perspective view of the projector used to provide anindicator for the densitometer.

Figure 8 shows a portion of spectrogram made according to my invention.

Figure 9 is an enlarged view showing the portion of the screen havingthe slot therein to re ceive the light passing through a portion of a'spectral line.

Figure 10 illustrates an opacity-exposure or light quantity curveobtained by the employmentof my method.

Figure 11 shows a typical master curve indicating the relation betweenpercent of one of the elements and the corresponding ratio of exposuresor light quantities.

Figure 12 is a horizontal sectional view showing a modified form ofdensitometer.

Figure 13 illustrates diagrammatically the amplifying apparatus usedwith the densitometer of Figure 12.

Figure 14 is a section on line |4l4 of Figure '12.

Figure 15 shows a modified method of mounting the photographic plate inthe spectrograph.

Figure 16 indicatesa stepped optical wedge" which may be used in placeof the stepped disc of Figure 4.

In-general, my method follows previous practice in that I select forstudy a pair of spectral lines, one of a base material, and the other ofthe material to be measured. I study these lines by making photographsof them, measuring the opacities of the photographed lines andconverting the measured opacities into the corresponding exposures orlight'quantities to obtain the desired light quantity or intensityratio. For the manner of selection of the spectral lines reference mustbe made to the voluminous literature on spectroscopy.

To obtain a photograph of the portion of the spectrum selected I haveused the apparatus as shown in Figure 1.

As a source of light I have employed an arc indicated at Ill. Thecathode or negative electrode i2 is shown in Figure 5. It is made ofspectrographically pure carbon or graphite, and is provided with a holeor socket l4 within which are placed particles of the material to betested. It was found that the shape of the end of the cored electrodehad an important effect on the steadiness of the are. In the case ofanalyses of cast iron for silicon content it was foundbest to give theend of the electrode a hemispherical shape. When a conical shape wasmade use of the arc was very unsatisfactory. It is indicated that thecurvature necessary to be given the end of the arc will vary inaccordance with the strength of the current and the size of the hole inthe electrode. In the case of the iron analyses mentioned above Iemployed holes of approximately in diameter, thereby being able to uselarger fragments ofthe metal without special grinding.

It may also be found desirable to employ neureflected from the silveredface 48, again passes ionization, and the fact that the electric fieldis a constant determined by the materials in the electrodes and is verysteady and reproducible. 10

For details of the method of arc control, which I found useful,attention is called to an article by R. Mannkopfl and C. L. Peters,contained in the July, 1931, issue of Zeitschrift fiir Physik.

While I have described the preferred method of l5 obtaining light fromthe material to be tested it is to be understood that other means ofexcitation may be employed such as the spark, flame, or gaseousdischarge in tubes.

Light from the region of cathode fall is projected on the slit of thespectrograph by the use of a suitable optical train to enlarge the imageof the region to workable sis'e, together with a screen which serves toexclude other light. For the optical train I have preferred to use alens 25 I6 having one focus at the arc and the other focus at anaperture in screen l8 which excludes all but the light desired, togetherwith a second lens .20 which forcuses the light on the slit 22 of thespectrograph.

The spectrograph itself is conventional, and may consist of a suitablehousing indicated by dot and dash lines in Figure 1, provided at one endwith suitable devices for adjusting the width and length of the slit 22.I have indicated a frame 24 35 provided with a guideway 26 within whichare slidably mounted'the slide 28 provided with a stud 30 projectingthrough an aperture in the rear of the frame 24 and yieldingly urgedtoward the left as illustrated in Figure 3 by means 40 of leaf spring 32engaging the stud 30 on the slide and studs 34 on the frame 24. Anadjusting screw 36 is threadedly mounted on the guide 24, and engagesstud 30 at one end. This screw may be provided with a graduated knob 38,and 45 the slide 28 may be provided with an indicator as shown forindexing the adjusting screw. By moving the slide 28 back-and forth thewidth of the slit 22 may be adjusted.

There is also mounted within the guideway 26 50 Y 42, passes throughlens 44, and through prism 46. having its rear face 48 silvered. Thelight is through lens 44, and is projected upon photographic plate 50mounted in the spectrograph, preferably in the manner shown in Figure15.

The prism 46 analyzes the light into its component rays in known manner,the result being an image of the spectrum, or rather of a portion of thespectrum, on the photographic plate 50. The lens 44 and prism '46 may bemounted for adjustment in the manner shown. This arrangement of prism 46is known as Littrow mounting, 70

and is well known in the art.

I have preferably provided a shutter for intercepting the light passingthroiigh the slit to control the p riod of exposure. .The shutter isindicated at 52, and may be actuated in any suitable 75 and I55. Theseslots are so arranged as to resists of the sectored disc 58 shownin'detail in Figure 4. This disc may be driven by motor 60 through anysuitable gearing, and provision may be made for regulating the speed asdesired. Itshould be capable of rotation at high speed, for example, inthe neighborhood of several thousand R. P. M.

The disc 58 is provided with a stepped periphery as shown. Each of thesteps covers the same angular distance, and the steps are preferably ofequal height as shown. The distance between the lowest step and thehighest step is preferably equal to the height of the slit and the discis so arranged that when the highest step is opposite the slit, light isentirely cut off froin it.

' The disc effectively divides the spectrogram trans- Since the stepsare of equal angular extent this period of time is equal to two units ofexposure.

The periods of exposure of the remaining por-' tions of the slot arealso multiples of the period of exposure of the bottom portion so that aspectrogram II is obtained, as shown inFigure 8, consisting of fiveportions of graduated opacity corresponding to the graduated exposuretimes.

I have shown in Figure 15 a modified mounting for the photographic plate50. The mounting consists of a support I5I carrying fixed guides I52 andI53 between which the photographic plate or film 50' may be inserted.The guide I53 is opaque, and is provided with slots I54 ceivea few onlyof the spectral lines including those to be measured. A series of guidesI53 may be provided having slots corresponding to the spectral linesneeded for analysis of different materials. By selecting the properguide and mounting it in the plate holder, the operator thereafter needdo nothing but insert the plate in position whereupon the proper lineswill be photographed. This greatly simplifies the operation. g

It will be noted that the photographic plate 50 of Figure 1 as well asthe plate 50' of Figure 15 are curved. This is because the spectroscopearranged as shown, brings different lines to a focus at differentdistances from the lens, in

known manner.

I have found it desirable to use photographic plates-of such type thatthey may be developed and fixed with great rapidity since speed ofanalysis is desired. The following kinds ofphotographic plates haveproven to be especially desirable for this use: Printon plates made byAgfa Ansco, Contrasto plates made by Difender Photo Supply Co.. andKodalith plates made by Eastman Kodak Company. In practice I prefer touse film rather than glass plate, but it will be understood that wherethe term plate is employed in the specification or claims I have in mindsimplya supporting surface for the light sensitive emulsion whatever bethe material of which that supporting surface is made.

The lines of the spectrograms as shown in Figure 8 have the appearanceof tapering in width due to the effect of halatiom However, each of theportions of stepped opacity will be found to contain a central strip,parallel to the length of the step, having uniform opacity.

It has been suggested that in place of the stepped disc indicated inFigure 4 a stepped wedge such as shown in Figure 16 may be used, the

wedge consisting of such material as will have as nearly as possible thesame absorbing power for light of all colors and cutting down thequantity of light as the steps increase in thickness by amounts lost byabsorption. Since the thickness of succeeding steps are multiples of thethickness of the first step a similar gradation of'opacities isobtained. I regard this as simply an inferior variant of my stepped discmethod.

The next step consists in measurementof the opacity of selected spectrallines. While these lines may be lines of the same chemical element, Iprefer to employ a line of a base material together with a line of theelement to be measured. Thus-in the case of iron used in casting brakedrums and other articles where it was desired to control the siliconcontent with great accuracy I employed a silicon line with an iron baseline.

- 'It is to be understood, of course, that I am using analysis of ironfor silicon content simply as one example, and that my method is ofbroad utility, and may be used for the analysis of any kind of materialpractically regardless of its physical form, provided the-element, orelements to be measured produce spectral lines.

To measure the opacities of the spectral lines I may employ any suitableapparatus, but have found it most convenient to use the densitometer ofspecial'design hereinafter described. In designing this densitometer Ihave borne in mind the need for simplicity so that the measurements maybe made by an unskilled operator.

My densitometer consists essentially of apparatus for projectingenlarged images of the spec-' tral lines on a suitable screen providedwith an aperture or apertures through which light passing through aportion of any desired line may be projected onto the light sensitiveelement of a light measuring device, such as a thermocouple,

a radiometer, a bolometer or a photoelectric cell.

I have had particular success with the Westor r photronic cell whichrequires no amplification.

A laboratory setup of my apparatus isv shown in Figure 6. Here 64indicates any suitable type of projecting apparatus provided with asource of light 66 of any suitable type. In order to secure'uniformityin results it is essential that the voltage on the lamp 66 be maintainedconstant. For this purpose I may use storage batteries to supply currentto the lamp, and may, if desired,

To make possible the use of cheaper lenses in the projector and ease ofadjustment I have increased the distance of projection withoutincreasing the length of the apparatus by employing mirrors l2 and 14 inseries to reflect the image on a screen 16. By using the mirrors thescreen is brought nearer to the eye of the operator so he can moreaccurately and quickly adjust the line on the slot. Screen 16 isprovided with a slot I8 scale 88.

With the desired arrangement the reading on the scale 88 is a measure ofthe amount of light passing through the section of the line of thespectrogram. To permit measurement of the successive sections of theline I have provided the projector with an elevating screw 94. I haveindicated at 96 an adjusting screwto move the imagelaterally on thescreen to bring other spectral lines over the slot.

In carrying out my process the apparatusshown in Figure 6 is used tomeasure the'opacity of each of the stepped segments of the iron line,and for convenience the results may be plotted against exposure or lightquantities as shown in Figure 10. The resultant curve shows the relationbetween opacity and the quantity of light in the spectral line producingthe image on the plate and serves as a calibration curve for thatparticular plate.

At the same time that the opacity of the iron line is measured withtheapparatus of Figure 6 the opacity of one section of the selected siliconline is likewise measured. An iron line is chosen .as standard in thiscase because there is so much iron present in all irons and steels thatthe intensities of its lines are constant for all samples. In othermaterials the same idea is carried out. I prefer to measure the opacityof the middle section. In the illustration given, the opacitymeasurement was found to be six, and by applying this measurement to thecurve of Figure 10, a value of approximately 2%; is obtained for thecorresponding light quantity. The light quantity for the middle segmentof the iron line is three since the shutter shown in Figure 4 isdesigned so that the period of exposure in the. case of the middlesegment is three times as long as in the case of the bottom segment.Therefore the ratio of light quantities of iron to silicon is The nextstep consists in evaluating this ratio in terms of silicon content. Toaccomplish this it is necessary to prepare a series of specimens ofknown silicon content, make a spectogram of each and obtain thecorresponding ratio of light quantitles ofthe selected lines in themanner just described. The results are plotted in Figure 11.Interpolating the ratio of light quantities for the unknown on the curveof Figure 11, the silicon content is determined to be 3%.

In general, it is not essential to my process that the series ofspecimens be identical. It is merely necessary that the variations intheir content are not sufficiently wide as to seriously alter therelations between the spectral lines. For example, specimens may be usedcontaining elements not present in other specimens of the same'series.Nor is it essential that thephysical forms of the specimens beidentical, for some may be alloys, while others may be conglomerates orphysical mixtures.

In Figures 12 to 14 I have shown an alternative of the spectrogramemploying a quick acting I variations in light.

photoelectric cell together with an amplifier to increase the currentfluctuations produced by The projecting lantern 64 may be the same asthat previously described with vertical adjusting screw 94 andhorizontal adjusting screw 96 to enable measurement of successivesegments of adjacent spectral lines. The projector throws the spectralpattern on screen I00 as before. This screen is provided with slot I 02to receive the image of a portion of a spectral line and with a secondslot I04. The slots I02 and I00 lie along radii from a common centerI06. There is rotatably mounted about the center I06 a disc I08-havingteeth on its periphery made in the form of a sine wave. The slots I02and I04 are spaced by a distance equal to one-half of the pitch of theteeth. The slots are preferably slightly longer than the disc teeth. Thedisc I08 is driven by means of an electric motor I09. The light passingthrough the slots is condensed by means of lens I I0, and smallimages ofthe source through the two slots are focussed on the target of thephotoelectric cell II2. In my preferred construction this condensedimage is but in diameter so that only a small portion of the target sistof two vacuum tubes coupled by a transformer. The current from thephotoelectric cell H2 is fed to the grid of the first tube. Theamplifier may feed into a vacuum thermocouple heater, indicated at II6,the thermocouple of which is connected to a microammeter I I8.

I have indicated in Figure 13 a conventional circuit which may be usedfor amplification, but it will be understood that this is subject toconsiderable variation in practice.

This form of densitometer has the advantage that the errors due tovariations in background, such as fogging, are greatly reduced. Thisreduction results from the fact that the light falling onthephoto-electric cell comes through two slots I02 and I04, each of whichis subject to the same errors due to fogging. With the constructionshown, if both slots are equally illumivariations in current. When theimage of a portion of aspectral line is projected on the slot I02 therotation of the disc I 08 produces variation in the amount of lightfalling on the cell H2. The two sine waves of current produced in thephotoelectric cell circuit by the action of the light through the slotsno longer counteract one another as the intensity through one slot isdecreased by an amount equal to the density of the projected line.The-overbalancing sine wave of current is amplified, and its proportionis read on the microammeter. The heaviest spectral lines produce thegreatest amount of overbalance. The frequency of the overbalancingcurrent is equal to the frequency of the teeth passing the slots, this,In one case, being about 300 cycles per second. By using a loadingresistor on the secondary of the coupling transformer the sensitivitycan be controlled at will. Up to the present time it hasnot been foundnecessary to use the highest sensitivity attainable with the two stageamplifier, even for the faintest lines studied.

With this arrangement the effects of variations in background areeliminated since it is the dif- 7 slots that is measured. This methodlikewise has the advantage of permittingeasy amplification because analternating current is obtained.

With this type of apparatus it is necessary to use a fast photo-electriccell. I have found the General Electric Companys high vacuum cell to bevery satisfactory.

With the type of densitometer last described the data of Figures 10 andll may be obtained;.

this modification simply representing a variation I in the methodof'measuring the opacities of the lines.

It will be appreciated that my method of quantitative spectroscopicanalysis is especially adapted for use in production. The userwill beprovided with the spectrographic equipment described, and with asuitable densitometer, and with a set ofmaster curves applying to thecompositions with which he is dealing. To make an analysis it is butnecessary to take a sample, insert it in the recess in the electrode,produce the arc, and take a photograph of the desired portion of thespectrum. The photograph may be developed, fixed and dried in less thana minute and placed in the projection apparatus to measure the opacitiesof the sections of the line of the base material to obtain the relationbetween opacity and light quantity or intensity; and to measure theopacity of one section of the element under test. From this the ratio oflight quantities or intensities may be obtained, and this ratio may beevaluated in terms of the element by reference to a master curve.

All of the operation may be performed by unskilled workers but at thesame time greater ac curacy is obtained for the reason that the methodof calibrating the plate is more accurate in that the calibration markeris built into the spectrum itself by varying the exposures of differentportions of the spectral lines.

arising from variation in wave length of the marker and of the linesmeasured, as well as the added complication of a separate calibrationmarker is avoided.

While I have employed a graduated calibration marker with fivegraduations, it will be understood that this has been done forconvenience only, and that the number of graduations may be. increasedto infinity, if desired, thereby obtaining a complete opacity-lightquantity or intensity of exposures, but since the times of the exposureand the areas exposed are the same, the .result- .ing ratio is also theratio of intensities of the spectral lines in the source.

It will likewise be apparent that, if preferred my plate calibrationmarker may be applied sep- By building the. calibration marker into thespectrogram. errors arately so that from one point of view my inventionconsists of a method of calibrating photographic plates for any, or all,portions of the spectrum by applying a-marker characterize by variationin exposures. V

' I claim:

1. The method of quantitative spectroscopic analysis which comprises thefollowing steps;

causing the material to emit light; producing the spectrum of themateriala recording on a photographic. plate a portion of the spectrumincluding a line of the unknown element and a reference line; graduatingthe time of exposure throughout the length of each of the lines in stepsof known ratio so as to graduate in like manner the quantity of light towhich each portion is subjected; measuring the. opacities of thedifferent portionsating the same in terms of light quantities byreference to the said scale; setting up a ratio of the.

light quantities forming corresponding portions of the line of theunknown and of the reference line: repeating the process with a seriesof stand- .of the reference line to obtain a scale of opacitiesi interms of light quantities; measuring the opacity of a portion of theline of the unknown; evaluard electrodes containing thesame kind ofmaterial but having the unknown element varied in known amount;evaluating the ratio of light quantities obtained from the unknowninterms 1 the spectral line of the unknown and the reference line lyingin substantially the same portion of the spectrum.

3. The method of quantitative spectroscopic analysis which comprises thefollowing steps; causingthe material to emit light; producing thespectrum of the-material; recording on a photographic plate'a portion ofthe spectrum including a line of the unknown element and a referenceline; graduating. the time of exposure throughout the length'of thereference line in definite steps of known ratio so as to graduate inlike manner the quantities of light to which each portion of the line issubjected, one of said steps corresponding with the time of exposure ofa portion of the line of the unknown element,' measuring the opacitiesof the different portions of the reference line to obtain a scale ofopacities in terms of light-quantities; measuring the opacevaluating thesame in terms of light quantities by reference to the said scale;setting up a ratio of the light quantities forming said portion of theline of the unknown element and the corresponding portion of thereference line; repeating the process with a series of standardelectrodes con-' taining the same kind of material but having theunknown element varied in known amount evaluating the ratio of lightquantities obtained from the unknown interms of percentage of thesubstance by comparison with ratios'of light quantities obtained fromthe standard series. 1

EDWARD J. MARTIN.

pity of said portion of the line of the unknown;

